Modular power supply system with control command verification

ABSTRACT

A modular power supply system ( 200 ), which comprises at least two power units ( 101′, 102′ ) connected in parallel for producing a certain ouptut voltage (DC) of a certain input voltage (AC), and a control unit ( 106 ) for giving control commands concerning the level of the output voltage of the power units to the power units. The control of the power units comprises steps in which: the control unit gives a control command to the power unit; the power unit examines whether the control command received by it meets certain saved criteria, whereby the control command which meets the saved criteria is executed ( 302, 303 ), and the control command which does not meet the saved criteria is not executed ( 301 ).

The invention relates to controlling the operation of power supplysystems in general. In particular, the invention relates to controllingthe modules of such a modular power supply system, in which acentralized control unit can make the modules operate at differentvoltage levels by commands.

FIG. 1 depicts a prior art modular power supply system 100, which isused to feed battery-backed electric power to the load. The systemcomprises N parallel power units, of which only 101 and 102 are shown inthe figure for clarity. The positive integer N is generally selectedsuch that even if one power unit failed, the performance of theremaining power units would be sufficient to meet the power requirementof the load in normal circumstances. The power units are rectifier andcontrol units, the inputs of which are connected to alternating current(AC), which is generally a common electricity distribution network. Theoutputs of the power units are connected to the load 103. The powerunits can be called the modules of the system.

In order to ensure uninterrupted power supply, the system includes astorage battery 104, which is connected in parallel with the load bymeans of the equipment 105, which prevents deep discharge of thebattery. The latter is essentially a switch, which cuts the supply ofelectricity from the storage battery to the load when required, and thusprevents a complete discharge of the storage battery in a situation inwhich the power units have been unoperational for some time, and thecharge of the battery has been mostly discharged to the load. Theoperation of the system is controlled by a control unit 106. In order toimplement data transfer between the control unit and power units, thesystem has a data transfer connection 107, which can be a serial bus,for example. In the following, the data transfer connection between thecontrol unit and the power units is generally called a serial bus,regardless of how the connection has been implemented in a practicalembodiment. The data transfer connection between the control unit 106and the equipment 105 which prevents deep discharge of the battery, canbe implemented through the same serial bus or there can be a separateconnection for it, which is independent of the serial bus.

Measurements of the condition of the storage battery 104 often requirethat the battery is discharged at times. For this, the output voltage ofthe power units 101, 102 is lowered to a so-called support level. Afterthe measurement, the storage battery must be fully charged again, forwhich the output voltage of the power units 101, 102 is raised back tothe permanent charge level or to a somewhat higher quick charge level.Quick charge is used particularly if the discharge of the storagebattery has taken a long time. After some time, the output voltage ofthe power units must be lowered to the permanent charge level in orderto prevent the overvoltage from damaging the storage battery or theload. Regulating the output voltage to the support level, permanentcharge level or quick charge level takes place so that the control unit106 gives an output voltage regulation command to each power unit viathe serial bus 107. The regulation commands are numeric values orencoded commands, which are converted into numeric values in the powerunits.

In addition to the centralized output voltage regulation, the systemcomprises selective overvoltage control, which is implemented separatelyin each power unit. The selective overvoltage control interrupts theoperation of the power unit in question, if the output voltage of thepower unit exceeds a certain predetermined overvoltage limit.Selectiveness means that the operation of the power unit is onlyinterrupted if the power unit is loaded in an overvoltage situation,that is, current is detected at its output simultaneously with an outputvoltage which is higher than the overvoltage limit. The operation of theselective overvoltage control in a certain power unit is not dependenton other power units.

The reliability of a system according to FIG. 1 is substantiallyweakened by the possibility of a common-mode failure. This means thatthe operation of at least two power units is disturbed by a common causesimultaneously and in the same way. Generally, the reason for this isthat the control unit has given the wrong command, as a result of whichthe output voltage of all power units is too low or too high and/or itremains at the wrong level for too long a time.

It is an objective of the present invention to provide a method andequipment for controlling the operation of a system comprising powerunits in such a manner that the probability of a common-mode failure issmall. It is also an objective of the invention that the manufacturingcosts of the construction are low.

The objectives of the invention are achieved by saving certain criteriain the power units, on the basis of which criteria the power units candetermine whether the control command given by the control unit issensible. The operation of the power units is arranged such that they donot execute a control command which does not meet the saved criteria.

The method according to the invention is characterized in that itcomprises steps in which

the control unit gives a control command to the power unit,

the power unit examines whether the control command received by it meetscertain predetermined and saved criteria, whereby

a control command which meets the saved criteria is executed, and

a control command which does not meet the saved criteria is not carriedout.

The invention also relates to a modular power supply system, which ischaracterized in that it comprises means in each power unit for savingcriteria that determine the rationality of the control commands, andmeans for comparing the received control command to the saved criteria.

According to the invention, the operation of a modular power supplysystem comprises a configuration mode and an operation mode. In theconfiguration mode, certain criteria are saved in each power unit, bymeans of which criteria the power unit can determine whether the controlcommand received by it is sensible. The saving is called theconfiguration of the power sources, and it is carried out in a situationin which it is clearly known what each command given by a control unitmeans and what each value contained in the command refers to. Theconfiguration can be carried out under the supervision of anelectrician, for example. In the operation mode, the power units receivecontrol commands from the control unit in the same way as in the priorart system, but instead of executing the commands without questioning,each power unit compares the command received by it to the criteriasaved in connection with the configuration and executes only thecommands which are sensible as compared to the criteria.

Suitable criteria include the numeric value of the level of each outputvoltage (permanent charge level, support level, quick charge level etc.)and the duration of an output voltage level which differs from thepermanent charge level. The power unit checks whether the output voltagelevel referred to by the control command meets the level of thecriterion, and for how long the command referring to an output levelvoltage which differs from the permanent charge level has been in force.If the output voltage level referred to by the control commandcontradicts the saved criteria, or if an output voltage level whichdiffers from the permanent charge level has continued longer than thecriterion allows, the power unit returns its output voltage to thepermanent charge level or interrupts its operation.

According to the preferred embodiment of the invention, certain factorysettings or limiting values have been fixed in the programmable parts ofthe power units or at the “hardware” or component level, which factorysettings function as criteria for the criteria used by the system. It isnot possible for the user to save a criterion which would violate thefactory settings of the power supply. The overvoltage protection of thepower supply, for example, is a factory setting, whereby it is notpossible to save a value higher than the output voltage that triggersthe overvoltage protection as the criterion of the quick charge level.

In the following, the invention will be described in more detail withreference to the preferred embodiments shown by way of example and theaccompanying drawings, in which

FIG. 1 shows a prior art modular power supply system,

FIG. 2 shows a modular power supply system according to the invention,and

FIG. 3 shows a method according to the invention.

In the above description of the prior art reference was made to FIG. 1,and thus in the description of the invention and its preferredembodiments reference will be made mostly to the FIGS. 2 and 3. The samereference numbers are used in the figures for corresponding parts.

FIG. 2 shows a modular power supply system 200, in which the differenceto the above prior art system is the fact that each power unit 101′,102′ comprises a memory device 201, a comparator 202 and a clock 203.Certain criteria, which concern the acceptability of the controlcommands can be saved in the memory device 201. The comparator 202compares the received control command to the criteria and determineswhether the control command is in harmony with the criteria. The clock203 operates in connection with the comparator 202, and it determinesfor how long the power unit has been at a voltage level which differsfrom the permanent charge level. In addition, the figure schematicallyshows the control input 204 at each power unit. By means of a signalbrought to this input, the power unit can be set in a so-calledconfiguration mode, which differs from the normal operation mode. Thecontrol input need not be a separate input, but the setting in theconfiguration mode can also be made by means of a command given throughthe serial bus 107. The configuration mode differs from the normaloperation mode so that during it the commands given by the control unit106 via the serial bus 107 are interpreted as values of the criteria,which are saved in the memory 201, and are not used to change the valueof the output voltage of the power unit.

The system according to FIG. 2 operates so that under supervisedcircumstances, each power unit in turn (or all power unitssimultaneously) are set in the configuration mode, after which thecontrol unit 106 gives the values of the criteria via the serial bus,and the values are saved in the memory of the power unit beingconfigured. In the following, the permanent charge level of the outputvoltage is marked with FC, the support charge level by SL and the quickcharge level by BC. In addition, the overvoltage limit of the selectiveovervoltage control is marked with SOV, and the lowest allowed outputvoltage, which triggers the operation of the equipment which preventsdeep discharge of the storage battery, is marked with LVD. It is assumedthat the storage battery is a lead battery with 24 cells, and its normaloutput voltage, which corresponds to a full charge, is 54.00 V. Thussuitable criteria to be saved in the memory of power units during theconfiguration mode are, for example:

permanent charge marginal: 53.00 V≦FC≦55.00 V

quick charge marginal: FC≦BC≦SOV−2 V

quick charge time: T_(BC)≦24 h

support charge marginal: LVD+2 V≦SL≦FC

support charge time: T_(SL)≦10 h

Here T_(BC) means the time during which the output voltage of the powerunit is at the quick charge level, and T_(SL) means the time duringwhich the output voltage of the power unit is at the support level.

When the configuration has been carried out, the power unit is set tothe normal operation mode by releasing the control input 204 or bygiving a command which denotes the normal operation mode via the serialbus 107. In the normal operation mode, the power unit receives commandsreferring to the level of the output voltage from the control unit 106,but instead of executing the received command immediately, it examinesby means of the comparator 202 whether the received command is such thatit can be executed without violating the saved criteria.

In addition to the criteria saved in the configuration mode, the powersupply can contain factory settings, which set limits to the kind ofcriteria that can be saved in the configuration mode. The factorysettings can be saved in the same or a similar memory device as used forsaving the criteria, or the component construction of the power supplyis such that it cannot apply a criterion which is on the wrong side ofthe factory setting. The latter alternative is generally described bysaying that the factory setting is built in the hardware of the powersupply. The factory settings ensure that the power supply cannot beconfigured wrong. The factory setting can function either so that acriterion which violates the factory setting is not saved at all, or sothat the criterion is saved, but if it exceeds the factory setting, thefactory setting and not the criterion is applied in the operation mode.

The mode diagram shown in FIG. 3 illustrates the operation of the powersupply. The basic mode of operation is 301, in which the power supplyoperates normally and its output voltage is at the permanent chargelevel. Transfer to operation at the support charge level 302 takes placeas a response to the command according to the criteria received from thecontrol unit, which is called the BL command in the figure. Return tothe basic mode 301 takes place either as a response to the FC commandaccording to the criteria or to an observation that the support chargetime specified in the criteria is expiring. The latter situation is asign of an error in the operation of the control unit or serial bus,because when operating correctly, the control unit must return theoutput voltage of the power units from the support charge level to thepermanent charge level (or the quick charge level) before the expiry ofthe support charge time. The error can be such that the control unit orthe serial bus is stuck in a mode which as such corresponds to a BLcommand which meets the criteria; so as to prevent the power unit fromchanging immediately back to the mode 302, it can be a good idea toinclude a timer in the mode 301 to prevent a quick return to the modefrom which the change to the mode 301 just took place.

It is also possible and correct to move from the mode 302 directly tothe mode 303, in which the output voltage of the power unit is at thequick charge level. A transfer like this takes place in the same way asthe transfer from the basic mode 301 to the mode 303, or as a responseto the BC command according to the criteria. Return from the mode 303 tothe mode 302 corresponds to return from the mode 302 to the mode 301,and thus it can take place as a response to an FC command according tothe criteria or to an observation of the expiry of the quick chargetime.

Transfer from the basic mode 301 to the configuration mode 304 takesplace when the power unit receives a configuration command via thecontrol input (or serial bus). In the configuration mode, reception ofthe control command does not cause changes in the output voltage but itcauses the received control commands to be saved as criteria inaccordance with the mode 305. Return from the configuration mode 304takes place as a response to the removal of the configuration command orto a separate command, which denotes return from the configuration modeto the basic mode.

The upper right part of the FIG. 3 shows the mode transfers, which arepossible in all the modes 301 to 305. The reception of a command whichviolates the criteria causes a transfer to the basic mode 301. Inaddition, the power unit comprises a selective overvoltage protection,prior art as such, which prevents the operation 306, if the outputvoltage exceeds a certain limit and current is simultaneously detectedat the output of the power supply.

The criteria saved by the power unit can be called variables, which areused particularly in control systems based on fuzzy logic.

What is claimed is:
 1. A power supply system (200), which comprises atleast two power units connected in parallel (101′, 102′) for producing acertain output voltage (DC) from a certain input voltage (AC), and acontrol unit (106) for giving control commands concerning the level ofthe output voltage, characterized in that it comprises in each powerunit means for saving criteria (201) which determine the rationality ofthe control commands (201), means for comparing (202) a received controlcommand to the saved criteria in order to determine the rationality ofsaid received control command, means for executing a received controlcommand that has been determined as rational through a comparison to thesaved criteria and a timer (203) for examining how long a certaincontrol command has been effective in order to enable time-outing theeffectiveness of control commands.
 2. A power supply system according toclaim 1, characterized in that it also comprises a control input (204)in each power unit for setting the power unit to the configuration modefor saving new criteria which determine the rationality of the controlcommands.
 3. A power supply system according to claim 2, characterizedin that it comprises a serial bus (107) for transmitting the controlcommands, to which serial bus the control unit and the power units areconnected, and said control input is the same as the connection of thepower unit to the serial bus.
 4. A power supply system according toclaim 2, characterized in that it comprises a serial bus (107) fortransmitting the control commands, to which serial bus the control unitand the power units are connected, and said control input is also aseparate input (204) in the power unit.
 5. A method for controlling theoperation of a modular power supply system, which power supply system(200) comprises at least two power units (101′, 102′) connected inparallel for producing a certain output voltage (DC) from a certaininput voltage (AC) and a control unit (106) for giving control commandsconcerning the level of the output voltage of the power units,characterized in that it comprises steps in which the control unit givesa control command to the power unit, the power unit examines whether thecontrol command received by it meets certain predetermined and savedcriteria, whereby a control command which meets the saved criteria isexecuted (302, 303), and a control command which does not meet the savedcriteria is not executed (301); and the power unit uses a timer (203)for examining how long a certain control command has been effective inorder to enable time-outing the effectiveness of control commands.
 6. Amethod according to claim 5, characterized in that it also comprisessteps in which the power unit is set in the configuration mode (304), acertain new criterion is saved in the power unit, and the power unit isset in the operation mode (301).
 7. A method according to claim 5,characterized in that when a control command which does not meet thesaved criteria is received, it is not executed but the power unit is setto operate in a certain basic mode (301).
 8. A method according to claim5, characterized in that it also comprises a step in which the operationof the power unit is prevented (306) as a response to a simultaneousobservation of overvoltage of the power unit and output current.